In the art of object detection, various "active" apparatus has been proposed which, for example, detect an object by projecting a beam of light (or IR) or sound and producing an output signal when this beam is interrupted to detect the presence of an object. Such a system requires two stations, the source (or a retroreflector) and a detector with a constantly generated source of light or sound. Some passive systems have also been proposed as, for example, a television scanner and memory where the outputs of the scene being viewed by the cameras are stored in the memory and this stored memory is compared periodically with a new scan. When an object moves into the area being scanned, the signals will not match those in the stored memory and an output will be produced indicating the presence of the object. These systems are fairly complex and quite costly.
In the art of range finding, particularly in auto focus cameras, a passive auto focus "module" is known. Various examples of such modules may be seen in the Stauffer U.S. Pat. No. 4,002,899 issued Jan. 11, 1977 which modules are quite accurate and low in cost. The auto focus module receives radiation from a scene being viewed and directs this radiation along two different paths to two similarly arranged detector arrays. One of the paths is usually made angularly variable and may cross the other path at various distances from the camera. When an object in the scene is located at the intersection of the two paths, then the radiation received by two detector arrays will be substantially equal and an in-focus condition is known to exist. On the other hand, when the object to be focussed upon is not at the intersection of the paths, the outputs of the detector arrays will be unmatched and an out-of-focus condition exists. By use of electronics disclosed in the above-mentioned U.S. Pat. No. 4,002,899, a correlation signal is obtained from the detector arrays and this correlation signal changes from a high level to a low level, or vice versa, as the ouputs of the detector arrays change from a matched to an unmatched condition. As used herein, the term "module" is intended to refer to the type of unit described above, that is, to a unit in which a pair of detector arrays receives radiation over a pair of radiation transmission paths from a scene being viewed and in which an output signal is produced that changes from a high state to a low state or vice versa when an object in the scene being viewed moves from a first range where the output of the detectors is substantially matched to a second range where the outputs of the detectors are unmatched.